Calibration mechanism for infrared space telescope

In order to achieve high infrared optical performances of the HELIOS2 telescope, a Calibration Mechanism, called MECALIB, is developed by ALCATEL. The specific kinematic architecture of MECALIB is based on an eccentric shaft, driven by a stepper motor, linked to a push rod that drives the rotative mirror mounting. This architecture presents the following main advantages: as the two pointing positions are close to the upper dead point of the mechanism, the reduction ratio is high enough to achieve the positioning accuracy during on ground tests (no need of zero G tools). the reduction ratio is highly non-linear so, even if the stepper motor is driven with full steps, the specified angle between the two pointing positions can be accurately adjusted by rotation of the motor housing. only preloaded ball bearings are used in the mechanism to achieve the function without any backlash. The development tests demonstrated that the MECALIB design meets all the requirements but a failure occurred during lifetime tests: the resistive torque increased drastically at the third of the specified lifetime. It has been investigated that the failure was due to the ball bearings MoS2 degradation. The design was modified to use fluid lubricated ball bearings. The qualification tests are in progress.